In this age of the growing importance of information, photochemical polymerization is now usually used in various fields, extending its application to the field of information storage and electronic circuit such as holography, lithographic plate for printing and printed- or integrated-circuit beyond the field of synthetic resin. Photochemical polymerization is a technique to bring polymerizable compounds into polymerization by light irradiation, which can be roughly classified into photopolymerization where polymerization is initiated by directly irradiating a photopolymerizable compound to activate it, and photosensitizing polymerization where a polymerizable compound is brought into polymerization by irradiating it in the presence of a photosensitizer to form active species of the photosensitizer. Each photochemical polymerization is characterized in that the initiation and termination of polymerization can be controlled by starting and stopping an exposing light source, as well as in that the degree and rate of polymerization can be easily controlled by selecting the strength and wavelength of the light source. Further, photochemical polymerization proceeds even at a reduced temperature because it usually does not require so much energy for initiating polymerization. In the field of information storage such as holography and lithographic plate for printing, there is an increasing demand for photo-polymerizable compositions which are polymerizable by irradiating them with a visible light such as the second harmonic of argon ion laser, helium ion laser, and Nd-YAG laser because of these advantages of photochemical polymerization.
Since many polymerizable compounds and polymerization initiators to be incorporated in photopolymerizable compositions absorb only ultraviolet ray, photosensitizers become one of key techniques in case that such a photopolymerizable composition is brought into polymerization with a visible light. The properties required for polymerization initiators are to be large in molecular absorption coefficient, capable of sensitizing various polymerizable compounds and polymerization initiators, high in sensitization efficiency, superior in solubility in solvents, mixing ability to other ingredients and stability. Typical organic compounds having a photosensitizing ability are, for example, melocyanine dyes disclosed in Japanese Patent Kokai No. 151,024/79, cyanine dyes disclosed in Japanese Patent Kokai No. 29,803/83, stilbene dyes disclosed in Japanese Patent Kokai No. 56,403/84, coumarin derivatives disclosed in Japanese Patent Kokai No. 23,901/88, methylene blue derivatives disclosed in Japanese Patent Kokai No. 33,103/89, and pyran derivatives disclosed in Japanese Patent Kokai No. 329,654/94, all of which however bear both merits and demerits: There have been found no substances which consistently exhibit the above-described properties in photopolymerizable compounds containing polymerizable compounds, polymerization initiators and binding resins. Thus, for example, in the field of information storage and electric equipment which are new application fields of photochemical polymerization, one first chooses several materials such as polymerizable compounds and binders which may meet to the final uses other than photosensitizers, then screens through trial and error a variety of organic compounds which may suit to the polymerizable compounds and polymerization initiators.
By the way, in the field of information displays, organic electroluminescent device (abbreviated as “organic EL device” hereinafter) has been highlighted as displaying device of the next generation. Currently, cathode-ray tube is predominantly used in information displaying equipments in larger sizes such as computer terminals and television sets. Cathode-ray tube is however large in both volume and weight and high in operation voltage, thus it may be unsuitable in portable equipments. More required is an information displaying equipment which is in a thinner and lighter panel form and operable with a lower voltage and less power consumption. At present, liquid crystal device is extensively used in various fields because of the merit that it is operable with a lower voltage and less power consumption. Liquid crystal device, however has the demerits that one can hardly receive a clear information therefrom when one view it at an angle outside the specific ranges, as well as that its power consumption is not so low as expected because it usually requires backlight. Organic EL device has been proposed as displaying device which may overcome the above demerits.
Organic EL device is a light-emitting device that utilizes a luminescence such as fluorescence or phosphorescence: It usually comprises a luminescent layer incorporated with a luminescent compound and inserted between a cathode and anode to which dc voltage is energized to inject holes and electrons in the luminescent layer so that a pair of hole and electron recouple each other to make in the luminescent compound an excited state which subsequently returns to the ground state to emit such a luminescence. Organic EL device is characterized in that its luminescent color tint can be controlled to a desired level by choosing an appropriate organic compound to be used as host compound in forming a luminescent layer, and screening a guest compound (or dopant) which may most suit to the host compound. Further, luminescent brightness and life expectancy may be remarkably enhanced, dependently upon the combination of host- and guest-compounds. Organic EL device has been deemed to be excellent in principle because of the merit that it emits a light in an autonomous manner and this would advantageously save power consumption.
Among luminescent compounds proposed hitherto, for example, those which have a coumarin skeleton have the merit that they are very safe and handleable with no special cares in organic EL devices because coumarin is a naturally occurring substance. Many conventional coumarin derivatives however exhibit a prescribed electroluminescence only when used in combination with host compounds, for example, quinolinol metal complexes, resulting in the problem that organic EL devices which may be practically useful with no obstructions can be hardly prepared by forming their luminescent layer with conventional coumarin alone or in combination with a guest compound.